Direct-sequence Code-division Multiple-access Cellular System Research Articles

Effects of Nakagami-Fading Parameters and Power Control Error on Performance of DS-CDMA Cellular Systems with Adaptive Beamforming

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) re...

Reverse Link Performance of DS-CDMA Cellular Systems through Closed-Loop Power Control, Base Station Assignment, and Antenna Arrays in 2D Urban Environment

The interference reduction capability of antenna arrays, base station assignment, and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. In this paper, we propose smart step closed-loop power control (SSPC) algorithm and base station assignment method based on minimizing the transmitter power (BSA-MTP) technique for direct sequence-code division multiple access (DS-CDMA) receiver in a 2D urban environment. This receiver consists of conjugate gradient adaptive beamforming and matched filter in two stages using antenna arrays. In addition, we study an analytical approach for the evaluation of the impact of power control error (PCE) on the DS-CDMA cellular systems. The simulation results indicate that the SSPC algorithm and the BSA-MTP technique can significantly improve the network bit error rate in comparison with conventional methods. Our proposed methods can also significantly save total transmit power and extend battery life in mobile units. In addition, we show that the convergence speed of the SSPC algorithm is faster than that of conventional algorithms. Finally, we discuss two parameters of PCE and channel propagation conditions (path-loss parameter and variance of shadowing) and their effects on the capacity of the system via some computer simulations.

DS-CDMA Cellular Systems Performance with Base Station Assignment, Power Control Error and Beamforming over Multipath Fading

The interference reduction capability of antenna arrays, base station assignment and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. In this paper, we propose base station assignment method based on minimizing the transmitter power (BSA-MTP) technique in a direct sequence-code division multiple access (DS-CDMA) receiver in the presence of frequency-selective Rayleigh fading and power control error (PCE). This receiver consists of constrained least mean squared (CLMS) algorithm, matched filter (MF), and maximal ratio combining (MRC) in three stages. Also, we present switched-beam (SB) technique in the first stage of the RAKE receiver for enhancing signal to interference plus noise ratio (SINR) in DS-CDMA cellular systems. The simulation results indicate that BSA-MTP technique can significantly improve the network bit error rate (BER) in comparison with the conventional case. Finally, we discuss on three parameters of the PCE, number of resolvable paths, and channel propagation conditions (path-loss exponent and shadowing) and their effects on capacity of the system via some computer simulations.

Reverse Link Performance of DS-CDMA Cellular Systems through Closed-Loop Power Control and Beamforming in 2D Urban Environment

The interference reduction capability of antenna arrays and the power control algorithms have been considered separately as means to decrease the interference in wireless communication networks. In this paper, we propose smart step closed-loop power control (SSPC) algorithm in direct sequence-code division multiple access (DS-CDMA) receivers in a 2D urban environment. This RAKE receiver consists of conjugate gradient adaptive beamforming (CGBF) and matched filter (MF) in two stages and finally, the output signals from the MFs are combined and then are fed into the decision circuit for the desired user. Also, we present switched-beam (SB) technique for enhancing signal to interference plus noise ratio (SINR) in network. Also, we study an analytical approach for the evaluation of the impact of power control error (PCE) on DS-CDMA systems in a 2D urban environment. The simulation results indicate that the convergence speed of the SSPC algorithm is faster than other algorithms. Also, we observe that significant saving in total transmit power (TTP) are possible with our proposed algorithm. Finally, we discuss two parameters of the PCE and path-loss exponent and their effects on capacity of the system via some computer.

Constrained Power Control to Enhance the Capacity of Slotted DS-CDMA Cellular Systems

We propose and study the constrained power control scheme for direct sequence code division multiple access (DS-CDMA) systems supporting voice and data services. In the proposed scheme, the voice users are subject to the conventional power control, while the data users are subject to distance dependent transmit power constraint. Several power control profiles for data users are suggested to reduce interference within its own and neighboring cells. For given delay and outage probability requirements, we present an analytical framework to analyze the number of users that can be supported by the proposed scheme. The results are compared to the conventional power control scheme, where no such constraint is imposed. We derive new closed form expression for the interference correction factor for the constrained power control scheme. In addition, we show that the total interference power can be approximated by the log-normal distribution, and we evaluate the mean and variance of the log-normal distribution analytically. We also derive the distribution of the signal-to-noise ratios for voice and data users, which is useful in evaluating the user capacity for slotted DS-CDMA cellular systems with both delay and power constraints.

Impact of Shadowing Correlation on Reverse Link Capacity of DS-CDMA Cellular System

Independent shadowing losses are often assumed for evaluating the link capacity of direct sequence code division multiple access (DS-CDMA) cellular system. However, shadowing losses may be partially correlated since the obstacles surrounding a mobile station block similarly the desired signal and the interfering signals. In this letter, we discuss how the shadowing correlation impacts the reverse link capacity of a power-controlled DS-CDMA cellular system, by numerical analysis.

A channel effect prediction-based power control scheme using PRNN/ERLS for uplinks in DS-CDMA cellular mobile systems

This paper proposes a channel effect prediction based power control scheme using pipeline recurrent neural network (PRNN)/extended recursive least squares (ERLS) for uplinks in direct sequence code division multiple access (DS-CDMA) cellular mobile systems. Conventional signal-to-interference (SIR) prediction-based power control schemes may incur prediction mistakes caused by the adjustment of transmission power. The proposed power control scheme purely tracks the variation of channel effect and, thus, can be immune to any power adjustment. Furthermore, it adopts the PRNN with ERLS for predicting the channel effect. Simulation results show that the channel effect prediction-based power control scheme using PRNN/ERLS achieves a 40% higher system capacity and a lower outage probability than the conventional SIR prediction-based power control scheme using grey prediction method (IEEE Trans. Veh. Technol., Vol. 49, No. 6, p. 2081, 2000).

Admission control scheme for soft handoff in DS-CDMA cellular systems supporting voice and stream-type data services

The soft handoff call requests of real-time services in third-generation (3G) direct-sequence code-division multiple access (DS-CDMA) and first- and second-generation cellular systems are more important than new call requests from the viewpoint of quality of service (QoS). Rejection of soft handoff requests causes forced termination of an ongoing real-time call, which is a severer problem than blocking of new call attempts. An admission control scheme that can guarantee a higher QoS for the soft handoff requests of real-time services in 3G DS-CDMA systems is proposed for delay-sensitive voice and delay-tolerant stream-type data services. The proposed scheme (P-Scheme) accommodates both voice and data services by utilizing the full bandwidth. However, voice soft handoff call requests are given priority over new voice call and stream-type data packet requests by suppressing interference from stream-type data services according to voice soft handoff requests, and by varying interference levels. Performance of the P-Scheme is evaluated using a Markovian model. Results are compared with a conventional reservation scheme (C-Scheme) that reserves resources exclusively for voice soft handoff requests. Numerical results show that system performance can be significantly improved using the proposed P-Scheme, compared with the conventional C-Scheme, when various types of service are supported in third-generation DS-CDMA systems.

Adaptive channel reservation scheme for soft handoff in DS-CDMA cellular systems

Soft handoff techniques in direct-sequence code-division multiple-access (DS-CDMA) systems provide mobile calls with seamless connections between adjacent cells. Channel reservation schemes are used to give high priority to more important soft handoff attempts over new call attempts. However, since the number of soft handoff attempts varies according to environmental conditions, fixed reservation schemes for handoff attempts can be inefficient. An adaptive channel reservation scheme is herein proposed to control the size of reservation capacity according to varying the number of soft handoff attempts. The proposed scheme also includes a balancing procedure between soft handoff failure and new call blocking to maximize the system capacity. To evaluate the performance of the proposed scheme, a Markovian model is developed that considers the interference-limited capacity effect of DS-CDMA systems. The analytical result shows that the proposed scheme yields a considerable enhancement in terms of new call blocking and soft handoff failure probabilities when compared with the conventional fixed channel reservation scheme.

Performance Evaluation for Forward-Link Cellular DS-CDMA overFrequency-Selective Nakagami Multipath Fading Channels

The bit-error rate (BER) for a forward-link cellular direct-sequence code-division multiple-access (DS-CDMA) system is evaluated. This analysis takes into account the effects of multi-cell interference resulting from Nakagami multipath fading, frequency selectivity, path loss and mobile user spatial distribution. Exponentially decaying multipath intensity profile (MIP) is adopted in the model to investigate its significance on the overall performance. In order to evaluate the BER performance efficiently, saddlepoint integration (SPI) is applied in the analysis. By fixing the number of resolvable multipaths and varying the number of fingers for the RAKE receiver, it can be shown that the capacity of a DS-CDMA cellular system increases almost linearly with the number of RAKE fingers.

On the analytical evaluation of closed-loop power-control error statistics in DS-CDMA cellular systems

This paper proposes an analytical study that aims at evaluating the power-control error statistics in wireless direct-sequence code-division multiple-access (DS-CDMA) cellular systems based on an ideal variable step closed-loop power-control scheme. In particular, the cumulative distribution function and the correlation coefficient of the power-control error are derived through a first-order Taylor expansion of the received signal envelope. A novel power-control scheme that exploits the autocorrelation properties of the fading is also proposed, and its performance is analyzed in terms of power-control error statistics. Rayleigh and Rice frequency-selective channel models, which involve the use of a diversity RAKE receiver at the base station, have been taken into account. The proposed analytical approach specifically applies to CDMA systems. A method that aims at estimating the capacity of a DS-CDMA cellular network is also given.

Hybrid channel assignment scheme for accommodating voice/data traffic in DS-CDMA cellular systems

A hybrid channel assignment (HCA) scheme in direct sequence-code division multiple access (DS-CDMA) systems for accommodating integrated voice/data traffic is proposed and the required power levels of voice and data traffic are derived. These levels can be used to maintain the minimum required link qualities of all calls. In the proposed scheme, delay-sensitive voice traffic is accommodated in circuit mode and delay-nonsensitive data traffic is accommodated in packet mode. The capacity region is derived and it can be used for controlling voice call admission and scheduling data packets. The proposed scheme can achieve a high link efficiency with reduced control overhead by statistically multiplexing voice and data traffic.

Interference-based channel assignment for DS-CDMA cellular systems

Link capacity is defined as the number of channels available in a link. In direct-sequence code-division multiple-access (DS-CDMA) cellular systems, this is limited by the interference present in the link. The interference is affected by many environmental factors, and, thus, the link capacity of the systems varies with the environment. Due to the varying link capacity, static channel assignment (SCA) based on fixed link capacity is not fully using the link capacity. This paper proposes a more efficient channel assignment based on the interference received at the base station (BS). In the proposed algorithm, a channel is assigned if the corresponding interference margin is less than the allowed interference, and, thus, channels are assigned adaptively to dynamically varying link capacity. Using the proposed algorithm yields more channels than using SCA in such an environment changes with nonhomogeneous traffic load or varying radio path loss. The algorithm also improves service grade by reserving channels for handoff calls.

Performance of closed-loop power control in DS-CDMA cellular systems

In situations where the round-trip delay between the mobile and the base stations is smaller than the correlation time of the channel, power control schemes using feedback from the base station can effectively compensate for the fast fading due to multipath. We study several closed-loop power control (CLPC) algorithms by analysis and detailed simulation. We introduce a new loglinear model for analyzing the received power correlation statistics of a CLPC scheme. The model provides analytical expressions for the temporal correlation of the power controlled channel parameterized by the update rate, loop delay, and vehicle speed. The received power correlation statistics quantify the ability of closed-loop power control to compensate for the time-varying channel. To study more complex update strategies, detailed simulations that estimate the channel bit-error performance are carried out. Simulation results are combined with coding bounds to obtain quasi-analytic estimates of the reverse link capacity in a direct-sequence code-division multiple-access (DS-CDMA) cellular system. The quasi-analytic approach quantifies the performance improvements due to effective power control in both single-cell and multicell DS-CDMA systems operating over both frequency-nonselective and frequency-selective fading channels. The effect of nonstationary base stations on the system performance is also presented.

Design of power control mechanisms with PCM realization for the uplink of a DS-CDMA cellular mobile radio system

Power control (PC) is an important issue in a direct sequence-code division multiple access (DS-CDMA) cellular mobile radio system. Higher link performance and greater system capacity cannot be achieved unless an appropriate PC mechanism is employed. In previous research, a delta-modulation (DM) realization of strength-based and SIR-based PC mechanisms for uplink communication has been studied by simulation. In order to obtain higher PC trackability, we study a pulse-code-modulation (PCM) realization of the above two PC mechanisms for the uplink of a DS-CDMA cellular mobile radio system. The simulation results presented indicate that the PC mechanisms with PCM realization for the uplink can achieve a lower outage probability and thus higher link performance than the PC mechanisms with DM realization. We also obtain optimal design parameters such as the stepsize and the control mode for the two PCM PC mechanisms. In addition, we compare the two PCM PC mechanisms in terms of their outage probability and stability and find that the strength-based mechanism has a higher outage probability but greater stability than the SIR-based mechanism.

Effects of radio propagation path loss on DS-CDMA cellular frequency reuse efficiency for the reverse channel

Analysis techniques for quantitatively describing the impact of propagation path loss and user distribution on wireless direct-sequence code-division multiple-access (DS-CDMA) spread spectrum systems are presented. Conventional terrestrial propagation models which assume a d/sup 4/ path loss law are shown to describe modern cellular and personal communication system channels, poorly. A two-ray propagation model and path loss model derived from field measurements are used to analyze the impact of path loss on the frequency reuse efficiency of DS-CDMA cellular radio systems. The analysis shows that the frequency reuse efficiency (F) of the reverse channel with a single ring of adjacent cells can vary from a maximum of 71% in d/sup 4/ channels with a favorable distribution of users, to a minimum of 33% in d/sup 2/ channels with a worst-case user distribution. For three rings of adjacent users, F drops to 58% and 16%, respectively. Using the two-ray model, it is shown that F can vary over a wide range of values due to the fine structure of propagation path loss.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>